Typically, empires attracted the top 10% of the peripheral/vassal nations. Look at Rome, for example. It is a sign of global standing that premier educational institutions of country X are flooded with the top performers of the other nations. Most importantly (and this is the reason this has been practiced by world powers throughout history) having the elite of a nation be educated in one's institutions guarantees a sizable cadre of influential men and women who go back home and create an 'Atlanticist' front.